The male fertility genes of Drosophila melanogaster will be examined in a restricted region of the X-chromosme, 18F-20EF. Preliminary studies indicate that there is an unusually high density of genes that are essential to male fertility in this region and many of the gene products function in the late stages of sperm development or during fertilization. Male sterile, female sterile and recessive lethal mutations will be isolated from EMS-treated X and Y ion Y mal ion chromosomes and localized in reference to each other by duplication and deficiency mapping, tests for allelism, and recombination. The sperm of male sterile mutants will be analyzed by cytological and cytochemical methods to determine the stage of sperm development or function that is genetically blocked. With this information it should be possible to determine: 1. if each male fertility gene is essential for a specific process in sperm development or function; 2. if male fertility genes, in some cases, facilitate a viatl function; and 3. if the genes for late sperm development are clustered in the proximal region of the X-chromosome.